The very first trailer for Knight and Day, the Cameron Diaz, Tom Cruise action movie that we showed you the poster for the other day is now online. With James Mangold (Walk the Line) directing the film, it looks to be a great action-comedy with both Cruise and Diaz looking like they’re having a good time.

Seeing these two actors together for the first time since Vanilla Sky, I’m reminded of the chemistry they had on-screen; and it’s fully evident again here. The trailer’s funny moments seem to be a continuation of Cruise not taking himself so seriously, much like in Tropic Thunder – which really helped to lighten the atmosphere around him, after all the Scientology and “Katie-gate” snafus.

Of course, the comedy meeting the action scenes make this seem like a perfect match for everyone involved. Diaz is showing her comedic side, such as she did in her hits, There’s Something About Mary and My Best Friend’s Wedding – where she plays much better as the ‘straight-man’, rather than trying to actually be a comedian. Mangold also comes through, giving us the action that he’s proven to do so well in films like 3:10 to Yuma and Copland. Put it all together and it’s a bundle of fun.

Since the plot of the film is “no one and nothing is what it seems – even themselves,” I’m sure there’s bound to be a twist in there that the bumbling character that Diaz seems to be playing will wind up actually being a trained assassin or something.

But, until we know for sure, check out the trailer courtesy of E! Online below, and let us know what you think. 20th Century Fox’s Knight and Day is scheduled for release on July 4th weekend, 2010.